The present invention relates generally to a superconductive magnet used to generate a magnetic field, and more particularly to such a magnet having magnetic shielding to protect the area around the magnet from stray magnetic fields originating from the magnet.
Superconductive magnets are used in various fields such as in MRI (magnetic resonance imaging) medical diagnostics. Known superconductive magnets are cooled by liquid helium or by a cryocooler coldhead. Conventional superconductive magnets include closed and open magnets. A closed superconductive magnet has a single superconductive coil assembly with at least one generally solenoidal-shaped main superconductive coil spaced apart from and surrounded by a generally annularly-cylindrical-shaped housing having a bore, with at least one thermal shield interposed between the main superconductive coil(s) and the housing. An open superconductive magnet has two longitudinally spaced-apart superconductive magnet assemblies with each superconductive magnet assembly similar to a closed superconductive magnet. It is noted that the open space between the two superconductive magnet assemblies of an open MRI medical magnet helps relieve the patient of any claustrophobic feelings and provides the doctor with access to the patient for surgical procedures during MRI operation.
Medical MRI superconductive magnets are magnetically shielded to prevent the high magnetic field created by and surrounding the main superconductive coil(s) from adversely interacting with electronic equipment located in the vicinity of the magnet. Known techniques for magnetically shielding superconductive magnets include active and/or passive shielding. Active shielding techniques include employing superconductive shielding coils which are cooled by liquid helium or a cryocooler coldhead and which are located within the housing. The superconductive shielding coils carry electric current of generally equal amperage and opposite direction to the electric current carried by the main superconductive coil(s). The total number of ampere turns of the superconductive shielding coils is equal to or greater than generally one-third of the total number of ampere turns of the main superconductive coil(s). The superconductive shielding coils are positioned within the housing radially outward of the main superconductive coil(s) so that the magnetic field of the superconductive shielding coils opposes the magnetic field of the main superconductive coil(s) to magnetically shield the area outside the magnet. It is noted that, due to the presence of the superconductive shielding coils, the total number of ampere turns of the main superconductive coil(s) must be made much larger to achieve the desired magnetic field strength in the magnet's bore (i.e., the bore of the housing), as can be appreciated by those skilled in the art.
Passive shielding techniques include employing iron in the outer cylindrical wall of the housing to capture and contain the magnetic fields generated by the main superconductive coil(s) so that such magnetic fields stay within the housing. A closed magnet would also employ iron in the two longitudinally spaced-apart end walls of the housing, while an open magnet would also employ iron in the longitudinally outer end wall of the housing for each of the two superconductive magnet assemblies. It is noted that an open magnet would further require an iron shield for the open space between the two superconductive magnet assemblies. However, closing the open space between the two superconductive magnet assemblies of an open magnet with an iron magnetic shield would eliminate the very advantages of the open magnet design.
Known superconductive magnet installations have included unshielded superconductive magnets sited in rooms having iron or resistive shielding coils located in the room walls to magnetically shield the areas outside the room.
What is needed is a superconductive magnet with improved magnetic shielding. The magnetic shielding must not require the increase in ampere turns for the main superconductive coil(s) or the use of extreme cooling measures as required when using superconductive shielding coils, and the magnetic shielding must not require the full size and weight of iron as required when using a conventional iron housing.